The invention is related to a closed press section in a paper machine, comprising a compact combination of press rolls in which the rolls form press nips with each other, between which nips the web has a closed draw supported by the face of a fabric, and comprising a center roll, in connection with which a press nip or press nips are formed and around which center roll a closed loop of a transfer band is passed, in whose outer face the web is transferred, after the last press nip in the compact combination of rolls, as a closed and constantly supported draw onto the drying wire or an equivalent fabric in the drying section following after the press section.
A problematic point in the prior art pres sections in paper machines is formed by the part in which the web that has passed through the nips in the press section is detached from a smooth-faced roll in the press, in particular from the center roll, being transferred to the drying section of the paper machine. This problem is accentuated with increasing running speeds of paper machines and/or when the paper web is very weak, e.g. when a maximal bulk is aimed at. In such a case, the pressing is carried out with very low nip loads, and the wet strength of the web remains low. The problem arises from the circumstance that, when the web is being pulled apart from the smooth-faced roll, a high tensile strain is applied to the web and that, when the web is being transferred from the press section to the drying section, it must run a short distance as a free and unsupported draw so that it is not supported by a roll face or by a fabric. When the running speed of the machine is increased, in such a case there is a high risk that the paper web is torn in said area. In conventional solutions, the web should preferably be detached from the center roll of the press section so that it runs to the drying section as straight as possible. Owing to the above, said area of transfer from the press section to the drying section has constituted a significant bottleneck in increasing the speed of a paper machine.
In prior art, attempts have been made to solve the problem discussed above, e.g., by means of the press sections described in U.S. Pat. Nos. 4,359,827 and 4,359,828, in which the web is not placed in direct contact with the face of the center roll in the press section, but a porous belt is fitted around the center roll, which belt corresponds to the length of the roll, i.e. to the working width of the machine, and which belt, guided by guide rolls, supports the web on said run of transfer from the press section to the drying section. However, it is a substantial drawback of the solutions of said U.S. patents that the porous band employed in these solutions loses some of its porosity in the nips in the press section, in which it is compressed to some extent. It is a second substantial drawback in these solutions that, at high temperatures, said band may lose most of its porosity, and in some extreme cases it may even melt.
As is known in prior art, attempts are made to employ high temperatures in the press section to intensify the dewatering. It is a further drawback in the solutions of said U.S. patents that the possibility of cleaning of the porous belt is very poor. In the nips in the press section, the pores in the belt tend to be blocked, and the U.S. patents do not suggest any means for conditioning and cleaning of the band.
In the assignee's FI patent application No. 885737 (filed Dec. 9, 1988), attempts have been made to provide a solution by whose means the drawbacks described above are avoided and an increased running speed of a paper machine is permitted and a problem-free transfer of the web from the press section to the drying section is ensured. In view of achieving the above, in said FI application, an endless metal band is passed over the center roll, which band is formed as a closed loop by means of guide and tensioning rolls, which loop is passed from the center roll to the beginning of the drying section, the web being arranged to be transferred from the pres section to the drying section as supported by said loop.
There is general aim to improve the dewatering capacity of presses in the press section of a paper machine. If the moisture content of a paper web can already be minimized in the press section, this amounts to considerable economies in the costs of paper manufacture, for the less wet the paper web is when it arrives from the press section, the lower is the consumption of energy in the drying section. It can be considered a rule of thumb that, if the moisture content of the web in the press section can be made lower by one percentage unit, the consumption of energy in the drying section is about four per cent lower, which means considerable economies in cost. The dewatering capacity is, as a rule, improved by raising the pressing temperature of the paper web.
In the constructions employed commonly in prior art, the center roll in the press constitutes an object of development. This is because of the material of the center roll, which is commonly some suitable rock, for example granite. It is, however, well known that rock rolls are quite sensitive to extensive and sudden changes in temperature, and the effects of such changes may be quite fatal. Attempts have been made to develop suitable substitutes for granite rolls. It is, however, difficult to make a suitable face for a center roll, and, moreover, the making of the face restricts the choice of the material for the rest of the roll. Also, different paper qualities require a different coating and frequently also a different process for the manufacture of the roll coating. Often it is necessary to manufacture different paper qualities out of different raw materials by means of the same paper machine. A change in quality would also require change of center roll or at least of its coating. A center roll is, however, an expensive and heavy component, and its replacement requires a long and costly standstill of the paper machine. If a center roll is provided, e.g., with a welded coating or if the coating is elastic, such as rubber-like, the device intended for heating of the paper web must necessarily be placed above the web if it is desirable to provide such a heating device in the construction before the third press nip. However, in a press, before the third press nip, there is a very scarce space available for an efficient device that raises the temperature of the web and regulates the temperature profile. However, it is a drawback of a heater placed in said location that it causes gathering of contaminations and their falling down onto the paper web.
An attempt has been made partly to solve the problems discussed above by means of the method and the device described in the applicant's FI patent application No. 891343 (filed Mar. 21, 1989). In said method, it has been considered novel that a transfer band, which has been formed as an endless loop by means of tension and guide rolls, is passed over the center roll in the press section, which transfer band extends to outside the area of the press rolls and onto which transfer band the paper web is transferred to run, and that the paper web is heated in the area of said loop outside the press rolls.
The device in accordance with the FI patent application No. 891343 does, however, not solve the problems discussed above and related to the detaching of the paper web from the face of the center roll and to its further transfer.
Thus, it is a general object of the present invention to provide a compact press section in a paper machine in which, at the same time, at least the most important ones of the problems discussed above are solved, together with some other problems, which will be dealt with in the following.
How to keep the smooth face of the center roll in a press section clean has become a problem in particular with fine papers. Further, a need has occurred to control and to regulate the length in the machine direction of the press zone in the roll nips formed in connection with the center rolls. This object cannot be achieved at all by means of a metal band running around the center roll (said FI patent application No. 885737).
One difficult problem, which was mentioned above preliminarily, arises from the fact that different paper qualities are often manufactured by means of the same paper machine, for example, depending on the market situation, the orders, or on the raw material that is available. The center rolls currently in operation and the belt solutions related to them have, however, not permitted a rapid and smooth change of quality. The change of quality ought to take place quite rapidly, because a standstill time causes considerable economic losses. Nor has consideration been given to quick replacement of the belt running around the center roll or of the other press fabrics and press rolls in the prior-art solutions. In the prior-art center-roll/transfer-belt arrangements, particular attention has not been paid to conditioning of the transfer belt running around the center roll or in other respects to safety and optimization of the transfer of the web taking place on said belt.